High-speed DSL (Digital Subscriber Line) equipment and associated communications lines are sensitive to crosstalk and characteristic impedance abnormalities of the DSL pairs. A particular DSL technology will typically suffer from these two abnormalities when the frequency content used by the particular DSL transceiver has a significant high frequency content. One example is very high bit rate DSL, Discrete Multitone (VDSL-DMT), which can have frequency content of up to about 30 MHz.
Multiple port, high-speed DSL products present a significant challenge in product design to minimize crosstalk between the multiple DSL pairs within the confined area of a printed circuit board (PCB) carried by the product. Typically, an electrical protection scheme, such as two fuses, is used for each DSL pair. One fuse protects one of the two wires, while the second fuse protects the other wire. These fuses are relatively large and usually not ideal for higher speed DSL applications. These fuses also generally cannot be placed on the printed circuit board to yield optimal differential characteristic impedance for each DSL pair. Moreover, the inherent spacing between the two fuses for each DSL pair makes it difficult to reduce crosstalk to/from the other DSL pairs in cases where the PCB restricts the area for DSL fuse placement. The large end caps typical to most fuses also compounds the crosstalk problem. These end caps act as capacitive plates that increase crosstalk coupling between adjacent pairs and reduce the characteristic impedance within each DSL pair. This makes it difficult to reach the normal characteristic impedance of 100 ohms of normal twisted pair wire. Thus, the spacing and design of fuses can become important in these communications applications.
In one particular example, a similar technical problem occurred when operating a TA3000 system manufactured by ADTRAN, INC. of Huntsville, Ala., under a particular set of circumstances. In this one particular example, the crosstalk and impedance mismatching became significant issues with the introduction of the VDSL line card. Prior to its introduction, the highest frequency content that was run over, the TA3000 backplane was 2.5 MHz associated with ADSL2+ line cards. With VDSL operating frequencies extending well beyond 2.5 MHz, the crosstalk dramatically increased. In addition, this system had long runs of DSL pairs across its backplane from the connection between the backplane line card connector to the backplane DSL pair connector. These long runs were relatively far away from the desired 100 ohm characteristic impedance needed for optimal VDSL operation.